Traditionally, polymers have been classified into two categories viz thermoplastics and thermosets depending upon their melting and solubility behavior. The thermoplastics on heating, are converted to a molten state and on cooling return to solid state reversibly. This property is made use of in shaping the polymers in various forms such as films, sheets, rods and other molded products. Also, these polymers are soluble in solvents and can be converted into films by solution casting and solvent evaporation. In contrast, the thermoset products cannot be converted into a molten state or dissolved in solvents reversibly. Although, these materials offer enhanced mechanical and thermal properties over the thermoplastics, they cannot be readily processed into finished products using processing techniques, commonly used in the case of thermoplastics. Similarly, the properties of the thermoplastics cannot be significantly enhanced after converting the resins into finished products since there is no scope to modify the polymer structure chemically after the polymerization is completed.
In few cases such as the phenolics, ureas and melamines, a two stage process is adopted whereby polymerization is first limited to a stage where the polymer can be fused into a molten state or dissolved in a solvent and then cross linked further into an infusible, insoluble product which has enhanced mechanical and thermal properties.
Thermosetting polymers containing reactive groups are used as coatings. These polymers are usually in the form of lattices that are further crosslinked either thermally or by addition of functional groups like isocyanates, amines or metal ions. By formation of a network, these resins attain their desired properties i.e., insolubility in most organic solvents, good water resistance and hardness (Van E. S. J. J. in Polymeric Dispersions: Principles and Applications. Asua, J. M. (Ed), Kluwer Publishers, 1997, p. 451; Ooka, M., Ozawa, H. Progress in Organic Coatings. Vol 23, 1994, p. 325). Photosensitive groups like cinnamoyl or azo type do not undergo thermal free radical polymerization but can be polymerized by UV irradiation. Polymers containing these functional groups can be cured by exposure to UV irradiation (Mueller, H., Mueller, I., Nuyken, O. Strohriegl P. Makromolecular Chemistry Rapid Communications, 13, 289, 1992; Raanby, B in Current Trends in Polymer Photochemistry. Norman, Allen (Ed), London, UK, 1995, p. 23). These materials can be used for non-linear optics.
In the case of unsaturated polyester resins, a polyester resin containing unsaturated sites is prepared by condensation polymerization using maleic anhydride and/or fumaric acid as the acid component. The resin, diluted with other vinyl monomers such as styrene, methyl methacrylate, allyl acrylate, etc. is cast into the desired form and then polymerized further to a crosslinked product in the presence of free radical initiators and accelerators/activators. While these resins are routinely used in the electrical and automobile industry, their scope is restricted. A large number of monomers such as styrene, methyl methacrylate, acrylonitrile, vinyl acetate, hydroxyethyl methacrylate, acrylamide and so on when polymerized by conventional methods of free radical polymerization result in solvent soluble melt fusible resins, which can be then converted to desired products. But as mentioned earlier, these products cannot be subsequently transformed into insoluble, infusible products, since there are no potential polymerizable sites present in the structure. On the other hand, copolymerization of these monomers with monomers containing multiple unsaturated sites viz Methylene bis acrylamide, Ethylene bis methacrylamide, Phenylene bis methacrylamide, ethylene glycol dimethacrylate, divinyl benzene, allyl acrylate, vinyl methacrylate results in the formation of three dimensional crosslinked products which cannot be further converted into useful forms since they are neither soluble in solvents nor be they can converted into a molten state on the application of heat.
Free radical polymerization of monomers comprising multiple unsaturated groups leads to insoluble polymers. There are few reports on the controlled polymerization of monomers containing multiple unsaturated groups using anionic polymerization. Thus, anionic polymerization of 1,4 divinyl or 1,4-diisopropenylbenzene led to reactive microgels containing pendant vinyl groups. But this method is restricted to divinyl compounds, which are amenable to anionic polymerization (Hiller, J. C., Funke, W. Angew Makromol. Chem., 76/77, 161, 1979. Wolfgang, S., Funke, W. Makromolecular Chemie, 179, 2145, 1978) and requires monomers of extremely high purity and very low temperatures for synthesis.
Recently Guan (Guan, Z. J. Am. Chem. Soc. 124, 5616, 2002), reported the synthesis of hyper branched polymers by cobalt mediated free radical polymerization of ethylene glycol dimethacrylate which resulted in a soluble poly (ethylene glycol dimethacrylate) polymer containing unsaturation. However, this method is specific to ethylene glycol dimethacrylate and cannot be readily extended to other monomers containing multiple unsaturated sites or to copolymers.
A wide range of cyclic compounds such as cyclodextrins, calixarenes, cryptands, and crown ethers are known to form host guest complexes and have been widely exploited commercially e.g. a number of drugs which are poorly water soluble and hence are poorly absorbed in the body have been encapsulated in the cyclodextrin cavity. The enhanced solubility leads to enhanced bioavailability of the drug. Crown ethers are macrocyclic polyether ring systems consisting of a number of oxygen joined by ethylene bridges. Crown ethers of 18-crown-6 type contain a cavity, which is able to form inclusion complexes with potassium, ammonium and protonated primary amines. The direct optical resolution of a number of di and tripeptides was achieved by capillary zone electrophoresis using enantioselective crown ether as buffer additive. (R. Kuhn, R. Daniel, F. Burkhard, W. Kart-Heinz. J. of Chromatography A, 716, 371-379, 1995). Chiral crown ethers are used to resolve enantiomers that contain primary amine functional groups. (D. W. Armstrong, L. W. Chang, S. S. C. Chang, J. of Chromatography A, 793, 115-134, 1998). Similarly, bis-tren cryptates represent a versatile class of receptors for anions both mono and poly atomic. They can impart selective affinity towards a variety of anions. (V. Amendola, L. Fabbrizzi, C. Mangano, P. Pallavicini, A. Poggi, A. Taglietti, Coordination chemistry, 219-221, 821-837, 2001). The potential of calixarenes as artificial receptors and sensors for biomolecules has long been recognized. Calixarenes owe their utility to their ability to act as host compounds, forming host-guest complexes in solution/(Lumetta, G. J.; Rogers, R. D.; Gopalan, A. S. ‘Calixarenes for separations, American chemical society: Washington, D.C. 2000). The selectivity for smaller alkali metals has led to their studies as sensor devices. (Rusin, O.; Kral, V. Sens. Actuators B, B76, 331-335, 2001 and Diamond, D.; Nolan, K. Anal. Chem., 73, 22A-29A, 2001).
Cyclodextrins are well known cyclic oligosaccharides that can solubilize hydrophobic compounds in aqueous media. (Wenz, G. Angew Chem. 106, 851, 1994). The solubilization is effected by complexation of the water insoluble species within the hydrophobic cavity of cyclodextrin. The use of cyclodextrin to dissolve suitable monomers in water has been described in the literature (Storsberg J., Ritter, H. Macromolecular Rapid Communications 21, 236, 2000, Jeromin, J., Ritter, H. Macromolecular Rapid Communications 19, 377, 1998, Jeromin, J., Noll, O., Ritter, H. Macromolecular Chemistry and Physics 199, 2641, 1998, Glockner, P., Ritter, H. Macromolecular Rapid Communications 20, 602, 1999). Some patents describe the use of cyclodextrin preferably in catalytic amounts in order to improve emulsion polymerization yields. (U.S. Pat. Nos. 6,225,299, 5,521,266).
The copolymerization of several N-alkyl methacrylamides with t-butyl methacrylate in water in the presence of methylated-β-cyclodextrin is described. (Ritter H., Schwarz-Barac S. and Stein P., Macromolecules, 36 (2), 318-322, 2003). Methylated β-CD was used to complex the hydrophobic monomers isobomyl acrylate and butyl acrylate yielding water soluble host/guest complexes. These inclusion complexes of monomers were polymerized in water and kinetics of polymerization was investigated. It was found that reactivity ratios of complexed monomer differ significantly from uncomplexed monomers and also molecular weight of polymers obtained from complexed monomers are higher than those obtained from uncomplexed monomers. (Glockner P., Ritter H., Macromol. Rap. Comm., 20(11), 602-605, 1999). The free radical polymerization of styrene or MMA in water with potassium peroxodisulfate as free radical initiator in the presence of randomly methylated β-cyclodextrin is described. This method gives the quantitative conversion of the monomers and leads to stable latexes with nearly monodisperse polymer particle size distribution without using any surfactant (Storsberg J., van Aert H., van Roost C. and Ritter H., Macromolecules, 36, 50-53, 2003). Hydrophobic methacrylic monomers such as t-butyl methacrylate, cyclohexyl methacrylate, and 2-ethyl hexyl methacrylate were complexed with methylated β-cyclodextrin. These complexes were polymerized in aqueous media using free radical initiation. (Madison P. and Long T., Biomacromolecules, 1, 615-621, 2000). Highly hydrophobic monomers cannot be readily incorporated by emulsion polymerization. The use of a catalytic level of cyclodextrin allows the use of very hydrophobic monomers in emulsion polymerization where cyclodextrin acts as a phase transport catalyst continuously complexing and solubilizing the hydrophobic monomers and releasing them to the polymer particles. (Lau W., Macromol. Symp. 182, 283-289, 2000) Free radical polymerization of complexes of N-methacryloyl-D, L-phenylalanine methyl ester derivatives focusing on enantiodiscrimination during polymerization in an aqueous medium is described. (Schwarz-Barac S., Ritter H., Schollmeyer D., Macromol. Rap. Comm., 24(4), 325-330, 2003). Emulsion polymerization of stearyl acrylate was carried out using cyclodextrin as a phase transfer agent. (Leyrer R., Machtle W., Macromol. Chem. Phy., 201, 1235-1243, 2000). The first example of the radical polymerization of a fluorinated 2-vinyl cyclopropane and its copolymerization with an alkyl 2-vinyl cyclopropane in an aqueous solution via their host-guest complexation with a randomly methylated β-cyclodextrin using a water soluble initiator 2,2′ azobis (2-amidinopropane) dihydrochloride is reported. (Choi S. W., Kretschmann O., Ritter H., Ragnoli M., Galli G., Macromol. Chem. Phys., 204, 1475-79, 2003). Methylated β-cyclodextrin was used to complex the hydrophobic monomers n-butyl acrylate, n-hexyl acrylate, cyclohexyl acrylate yielding the corresponding water soluble host/guest complexes. The copolymerization of uncomplexed monomers leads to nearly ideal statistical copolymers (Bernhardt S., Glockner P., Ritter H., Polymer bulletin, 46, 153-157, 2001). The polymerization mechanism of methylated β-cyclodextrin complexes of phenyl methacrylate and cyclohexyl methacrylate is described by Jeromin and Ritter. (Jeromin J., Ritter H., Macromol. Rap. Comm., 19, 377-379, 1998).
A survey of the prior art in the field of polymerization of complexes containing cyclodextrins reveals that the preparation of host-guest complexes comprising monomers containing multiple unsaturation and cyclic compounds has not been reported till date. It has been found that monomers which contain multiple unsaturation form inclusion complexes of varying stoichiometries with cyclodextrins. Further, the unsaturated sites encapsulated within the cyclodextrin cavity do not react with the growing free radical chain. The polymerization of inclusion complexes of vinyl monomers containing multiple unsaturation, therefore leads to soluble polymers containing unreacted unsaturated sites. Once cyclodextrin is removed from the system, the deprotected unsaturated site can participate in polymerization in the second stage and lead to crosslinked products having enhanced mechanical, thermal and solvent resistance characteristics. These polymers therefore, offer the ease of processing of thermoplastics and enhanced properties of thermosets.
Cyclodextrin has been used in the present invention not only for the dissolution of monomers in water; but also to prevent one of the unsaturation sites present in the crosslinker from taking part in polymerization. Physical interactions are always preferred over chemical modifications as these are readily reversible. The inclusion complexes of hydrophobic/hydrophilic crosslinkers have several advantages over the complexes of the monomers containing only one unsaturation. These inclusion complexes increase the solubility of the monomer and can be used for copolymerization with different monomers giving soluble polymers. The unsaturation sites present after polymerization can further be thermally/photochemically crosslinked to give insoluble polymers. Also, the method can be used to prepare polymers of different architectures.
The demand for environmentally benign processes is growing due to increasing awareness of environmental issues involving conventional organic solvents. The chemical industry is encouraged to look for new means to the same end for many of its traditional processes that either produce environmentally unfriendly industrial products or result in toxic by-products. In an effort to overcome such potential obstacles with minimal expense, research is directed towards the replacement of traditional organic solvents with environmentally benign compounds such as carbon dioxide, biomolecules, and water. Complexation with carbohydrate monomers increases the solubility of hydrophobic monomers and enables its polymerization in aqueous medium. These carbohydrates can be easily recycled after polymerization. In our copending application patent no. PCT/IB03/03593 cyclodextrin complexes with acrylates/methacrylates have been mentioned which have little solubility in water. Since these complexes are hydrophobic they are normally not suitable to synthesize water-soluble polymers. Hence, there is a need to synthesize complexes comprising hydrophilic crosslinkers, which can be copolymerized with hydrophilic as well as hydrophobic monomers.
Typical water-soluble crosslinkers are Methylene bis acrylamide (MBAM), Ethylene bis methacrylamide (EBMA) or Phenylene bis methacrylamide. These crosslinkers have widespread applications. MBAM improves the stability of the membrane in an oxidative environment, which shows that MBAM crosslinked styrene membrane should work well in a fuel cell environment (Becker, W.; Schmidt-Naake, G., Chemical Engg. and Technology, 25 (4) 373-377, 2002). Interpenetrating network of methacrylamide and MBAM is used for selectivity in ion sorption i.e Fe2+ sorption and Cr6+ rejection. (Chauhan, G. S.; Mahajan, S., J. Appl. Poly. Sc., 86(31), 667-671, 2002). Superabsorbents made from Poly (Acrylamide-co-2-hydroxymethyl acrylate) in the presence of MBAM and potassium methacrylate are used for water managing materials for agriculture and horticulture purposes as it retains more moisture for longer time. (Raju, K. M.; Raju, M. P.; Mohan, Y. M., J. Appl. Poly. Sc., 85(8), 1795-1801, 2002). Poly (2-Acrylamido methyl propane sulphonic acid) prepared in the presence of MBAM and benzophenone was found suitable for MIP membrane synthesis. (Piletsky, S. A.; Matuschewski, H.; Schedler, U.; Wilpert, A.; Piletska, E. V.; Thiele, T. A.; Ulbricht, Macromolecules, 33(8), 3092-98, 2000). Also, thermally stable water swollen gels are used for fluid diversion in petroleum production. (Suda, Makoto; Kurata, Tooru; Fukai, Toshihiro; Maeda, Kenichiro, J. Pet. Sci Eng., 26 (1-4), 1-10, 2000). When Polyacrylamide gels are prepared in the presence of MBAM, Ethylene glycol dimethacrylate, 1,4 butanediol diacrylate/diallyl phthalate, more water absorbency was observed when MBAM was used as a crosslinking agent. (Raju, K. Mohana; Raju, M. Padmanabha; Mohan, Y. Murali, Polymer International, 52(5), 768-72, 2003).
Water soluble monomers such as acrylamide, acrylic acid or N-vinyl pyrrolidone are normally used in the presence of crosslinkers for immobilization of enzymes. Poly (acrylic acid) prepared in the presence of a MBAM, benzyldimethyl ketal pyrrolidone carboxylic acid is used as bioelectrodes with low impedance between electrode and skin. (JP 09038057 and JP 09038057). Poly (acrylamide-co-N acryloyl para amino benzamidine) synthesized in the presence of MBAM is used as molecularly imprinted polymeric receptor for trypsin. (Vaidya A. A.; Lele, B. S.; Kulkarni, M. G.; Mashelkar, R. A. J. App. Poly. Sc., 81(5), 1075-83, 2001). Poly (NIPA-co-MBAM) can be used to detect HBV viruses and for the concentration of either nucleic acids/proteins. (Pichot, C.; Elaisari, A.; Duracher, D.; Meunier, F.; Sauzedde, F. Macromol. Symposia, 175, 285-397, 2001). Poly (NIPA-co-AA) hydrogel prepared in the presence of MBAM is used for concentrating aqueous dispersions of bacteria. (Champ, S.; Xue, W.; Huglin, M. B. Macromol. Chem. and Phys., 201(17), 2505-2509, 2000). Poly (Acrylamide-co-Na acrylate) synthesized in the presence of MBAM is found to be useful for immobilization of Saccharomyces cerevisiae enzyme. (Oztop, H. N.; Oztop, A. Y.; Karadag, E.; Isikver, Y.; Saraydin, D., Enzyme and Microbial Technology, 32(1), 114-119, 2003). Poly (NIPA-co-HEMA) prepared in the presence of MBAM is used in enzyme activity control, extraction and drug delivery systems. (Lee, W. F.; Huang, Y.L. J. App. Poly. Sc., 77(8), 1769-1781, 2000). However, in all these cases, the unreacted crosslinker, which is toxic, is difficult to remove from these swollen gels. (George D. J., Price J. C., Marr C. M., Myers B. C., Schwetz A. B. and Heindel J. J. Toxicological science 46(1), 1998, 124-133). Hence, if a polymer comprising MBAM be prepared, freed from unreacted MBAM, then crosslinked, thus will overcome one of the limitations of these polymers in intended applications. It is the objective of this invention to demonstrate synthesis of such polymers.